Conventionally, an automatic transmission is applied to an apparatus such as a vehicle. As disclosed in JP-A-2001-59570 (U.S. Pat. No. 6,357,289B1) and JP-A-2001-116134 (U.S. Pat. No. 6,375,591B1), a hydraulic control apparatus controls hydraulic pressure applied to frictional elements in an automatic transmission. Engagement and disengagement of each frictional element is controlled by the hydraulic control apparatus, so that a gear position is changed. Besides, hydraulic pressure, which is applied to each frictional element, is detected using a pressure switch, so that a failure is identified in accordance with a combination among detection signals of the pressure switches.
As shown in FIG. 7, one pressure switch detects hydraulic pressure applied to each frictional element in a hydraulic pressure control apparatus, in the same manners as those of the structures in U.S. Pat. No. 6,357,289B1 and U.S. Pat. No. 6,375,591B1.
Frictional elements include a reverse clutch (R/C) 1, a low-reverse brake (LR/B) 2, a low clutch (L/C) 3, a 2-4 brake (2-4/B) 4, and a high clutch (H/C) 5. The frictional elements are engaged, and disengaged in accordance with hydraulic pressure. Control means 11 to 15 respectively switch hydraulic pressure applied to the frictional elements 1 to 5. Each of the control means 11 to 15 is constructed of a solenoid valve and a spool valve, for example. The spool valve switches hydraulic passage in accordance with operation pressure of the solenoid valve. The manual valve 20 is connected with a select lever 30 via a wire or the like. The select lever 30 is operated by a driver to change a drive range (select lever position, gear position). When the drive range is changed, a manual valve 20 switches hydraulic pressure passages that are respectively connected to the control means 11 to 15. In this situation, hydraulic pressure, which is applied to each control means 11 to 15, is switched from one of line pressure and hydraulic pressure of a drain 22 to the other of the line pressure and the hydraulic pressure of the drain 22, in accordance with the drive range. Each of pressure switches 40 outputs detection signal of hydraulic pressure applied to each frictional element. The detection signal of the pressure switch 40 is an on/off signal that is switched on a threshold that is set at detection main pressure.
For example, a gear position is changed from the second gear to the third gear in the D range (drive range) in accordance with a table of engagement shown in FIG. 8. As shown in FIG. 9, when hydraulic pressure 204, 205 applied to the H/C 5 increases, the pressure switch 40, which detects the hydraulic pressure 204, 205 applied to the H/C 5, is turned ON, so that a dual-engagement can be detected. In FIG. 9, hydraulic pressure 205 applied to the H/C 5 increases faster than normal hydraulic pressure 204 applied to the H/C 5. Hydraulic pressure 200 is applied to L/C 3, and hydraulic pressure 202 is applied to the 2-4/B 4 that is disengaged.
However, in the structures disclosed in U.S. Pat. No. 6,357,289B1 and U.S. Pat. No. 6,375,591B1, one pressure switch 40 is provided to each frictional element 1 to 5, thereby, each pressure switch 40 can detect a condition of hydraulic pressure relative to one threshold. As shown in FIG. 10, when the 2-4/B 4 is not disengaged, hydraulic pressure 203 applied to the 2-4/B 4 is maintained high. In this case, when the threshold of the pressure switch 40 is set low, e.g., set at the first hydraulic pressure P1, a failure cannot be detected, or a dual-engagement may be detected after the dual-engagement arises.
Specifically, when hydraulic pressure applied to each frictional element 1 to 5 becomes greater than the first hydraulic pressure (low-threshold) P1 in FIGS. 9, 10, the frictional element engages. When at least two of the frictional elements simultaneously engage, a dual-engagement arises. The gear is changed on the point A in FIG. 10, and hydraulic pressure applied to the 2-4/B 4 is supposed to decrease as shown by the hydraulic pressure 202. When the threshold of the pressure switch 40 is set low, i.e., set at the first hydraulic pressure P1, disengagement of the 2-4/B 4 is detected on the point C in FIG. 10 by the pressure switch 40. In this case, if a failure arises, and the hydraulic pressure applied to the 2-4/B 4 is maintained high as shown by the hydraulic pressure 203, the hydraulic pressure 203 may be determined to be in a failure condition on the point C or later. That is, when the pressure switch 40 does not detect the hydraulic pressure 203 to be less than the first hydraulic pressure P1 on the point C or later, the hydraulic pressure 203 may be determined in the failure condition. Accordingly, the condition of the hydraulic pressure 203 may be determined at the point C or later after elapsing a long time from starting of the gear change on the point A. When the hydraulic pressure 203 applied to the 2-4/B 4 is maintained high, a dual-engagement arises on the point B in FIG. 10, on which the hydraulic pressure 204 applied to the H/C 5 increases over the first hydraulic pressure P1 so that the H/C 5 engages.
On the contrary, as referred to FIG. 9, when the threshold of the pressure switch 40 is set high at second hydraulic pressure P2, and the hydraulic pressure 205 applied to the H/C 5 increases faster than a predetermined speed, a dual-engagement cannot be detected even after the dual-engagement arises.
Specifically, the threshold of the pressure switch is set high, e.g., set at the second hydraulic pressure P2, and the hydraulic pressure 205 applied to the H/C 5 may increase faster than the predetermined speed. In this case, the hydraulic pressure 205 applied to the H/C 5 increases over the first hydraulic pressure P1 on the point B in FIG. 9, and the H/C 5 engages. Besides, the hydraulic pressure 202 applied to the 2-4/B 4 is still greater than the first hydraulic pressure P1 on the point B in FIG. 9, and the 2-4/B 4 still engages. That is, a dual engagement arises on the point B in FIG. 9. As time elapses, the hydraulic pressure 205 increases after passing the point B, and the hydraulic pressure 205 becomes greater than the second hydraulic pressure P2 on the point C in FIG. 9. At this point, increase of the hydraulic pressure 205 can be detected by the pressure switch 40. That is, the dual-engagement cannot be detected in the zone between the point B and the point C, when the threshold of the pressure switch is set high, e.g., set at the second hydraulic pressure P2.
Accordingly, when the pressure switch has only one threshold on either the low-pressure side or the high-pressure side for each frictional element, not only a dual-engagement but also a failure of hydraulic pressure cannot be detected on one of the engagement-side and the disengagement-side.